Frequency modulation



Nov. 3o, 194s.

G. L. USSELMAN FREQUENCY uonum'rron Fund nay 16,1944

3 Sheets-Shut l .faz/ecs Nov. 30 1948.

G. l.. UssELMAN K FREQUENCY MODULATION Filed 16, 19M

3 Skeeter-Sheet 2 Jol/acs mmvron 650265 Mme-L MA/v ,M/WW

Patented Nov. 30, 1948 FREQUENCY MODULATION George L. Usselman, Port Jefferson, N. Y., as-

signor to Radio Corporation of America, a corf poration of Delaware Application May 16, 1944, vSerial No. 535,828

9 Claims. (Cl. 179-1715) This application discloses an improved means for generating wave energy andy modulating the timing, that is, the phase or frequency of the generated wave energy in accordance with signals or control potentials. The signal potentials may represent voice or code, such as, for example, keyed impulses as used in frequency shift keying.

In general, in known frequency modulation sys tems the carrier deviation results from circuit reactance changes, and tube capacities which form part of the circuit reactance limits the amount of circuit reactance variation to thereby limit the possible carrier deviation. In other words, in these prior circuits tube capacities were tolerated or mitigated by compensating means but were not used as part of the variable reactance for modulating the timing of the generated oscillations. In the improved system of this application, Y I make use of the tube capacities to obtain a greater degree of frequency modulation.

An object then of my invention is to increase the range through which the frequency of a carrier can be deviated Iby control potentials or signals.

In many of the known frequency modulation systems, one or more carrier wave generator tubes are used, and one or more modulator tubes are used. In my improved system of the present application, a single tube carrier generator isarranged in a simple although highly eiiicient circuit to generate a carrier wave and to modulate the frequency of the generated wave by control potentials or signals.

A more detailed object of my invention then is increased deviation of the carrier frequency by control or signal 'potential in a simplified and efficient manner.

An additional object of my invention is to generate a carrier Wave, deviate its frequency in accordance with signal or control potentials, and conne the band of frequencies over or through which the carrier is deviated between selected limits as desired.

The manner in which the above objects and others are attained, will be described in detail hereinafter. In this description reference will be made to the attached drawings wherein Figs. 1 to 3, inclusive, each show a different embodiment of my improved timing modulation system. In Figs. 1 to` 3 the modulation is applied by means of a transformer. Figs. 4 and 5 illustrate keying means for the modulators when shift frequency modulation is to be used.

In Fig. 1, a tube VI has its anode 4 connected to one end of the tank circuit C ILI and its cathode grounded. The control grid 8 of the tube Vl receives feedback excitation through coupling and blocking condenser IIJ and phase shifting inductance L arranged to connect the control grid to a point on the inductance L1 of the tank circuit CILI. One end of the grid resistor R is connected to the tank circuit 'end of coil L and also to the direct current blocking condenser I0 and thereby to a point on Li. The other end of the resistor R is connected to the cathode of the tube VI. The grid resistor R by virtue of grid rectication supplies the bias potential for the grid 8. A source of biasing potential may be included between R and the cathode if more negative bias is required on the grid 8. A point intermediate the points of connection of the anode il and the grid 8 to the inductance Ll' is grounded for radio frequency by condenser yi. and thereby connected to the cathode..

Oscillations are generated in the system described by virtue of the fact that the anode t and grid 8 operate at opposite phases in the radio frequency circuit. The oscillations generated are of a frequency determined by the tuning of Ci and Ll which may be considered the tank circuit of the regenerative Wave generator.

The inductance coil L, the tube grid to cathode capacity C and the tube current resistance constitutes the phase shifting elements through which or over which the feedback excitation voltage reaches the grid 8 from the tank circuit It. The oscillations generated are varied in frequency by varying the phase of the excitation fed back from the tank circuit CIL! through inductance L to the grid 8. The radio frequency current taken from the tank circuit is shifted in phase as it flows through the coil L to the grid 8 of tube Vl and through the grid capacity and tube current to the cathode of tube Vl and to ground and through coupling and bypassing condenser 2 to a point on LI. The tube radio frequency current may be considered to be a variable resistance in parallel with the tube capacity C so that a variation of this tube current caused by the signal modulations, Varies the phase of the feedback excitation and thereby varies the frequency of the oscillations generated in accordance with the signals. In my improved system I make the inductive reactance of the coil L substantially equal to the reactance of the tube grid capacity; This provides optimum excitation voltage and phase shift. The grid leak resistance R. is cor.- nected on the tank circuit side of the phase shifting circuit so that R does not aiect the phase vide maximum excitation phase shift.

To vary the tube current in accordance with signals or control potentials, I connect a source of control potentials A to the primary winding of transformer T, the secondary windingof which is coupled in series with source B between the screen grid I2 and the cathode of the tube. The signals at A operate to vary the potential on the screen grid I2 in accordance with the signals or control potentials to thereby vary the frequency of the oscillations generated. Some amplitude modulation is encountered in the embodiment of Fig. 1, but this may be removed in additional stages which may be coupled to L2 to derive the output and use the same as desired. In an embodiment of the arrangement shown, dimensioned to operate at 1775 kc. mean carrier frequency, a deviation of 7 kc. frequency sweep was obtained. An 865 tube was used -in the test setup. The inductance L was about 1 millihenry. The resistance R was about 20,000 ohms. Condenser Ci and coil Lx were rnade to tune at 1775 kc. These values may vary somewhat without greatly affecting the operation of the modulator. The signals at source A were of the A. C. type such as a modulated tone or voice frequency. When this is applied to transformer T and the screen grid of tube Vi the tube current is varied. As explained before, a variation in tube current varies the tube resistance so that the phase of the R. F. excitation is varied, which produces frequency modulation in tank circuit CI-LL Consequently, the frequency of the oscillator is varied in accordance with theaudio signal frequency and amplitude variations. Some amplitude modulation is produced by Fig. l, but this is usually eliminated in the following transmitter stages.

Where telegraphy signals are to be used and the frequency of the system is to be shifted from one value to another for the mark and space conditions, the arrangement of Fig. 1 may be modified as indicated in Fig. 4. To do this, the leads t the electrodes of tube Vl, Fig. l, are broken at the points crossed and this tube and connections replaced by the tu-be and connections of Fig. 4. Now as the key is manipulated, resistances 22 and 26 through bias source B apply different values of direct current potential to the screen grid i2 to change the current to the grid from a first value'to a second value to thereby shift tne frequency of operation from a first value corresponding to say mark` (when key k is open), to a second value corresponding to space (when the key k is closed) The embodiment of Fig. 2 is similar to the embodiment of Fig. l, except as described in detail hereinafter. The inductance L in Fig. 2

is connected to the inductance Li through a bandpass filter comprising crystal X and a neutralizing element N. This band pass filter has the two branches N and X symmetrically tapped on opposite sides of the center point on Ll which is grounded for radio frequencies by condenser 2.

The plate direct current is also fed to anode 4- through this center point. The resistance R is connected to the junction point between inductance L and crystal X which now serves as a frequency stabilizing element, a band pass lter,

coupled in series with source S2 between the screen grid i2 and cathode of tube Vi. The other secondary winding has one end connected to the cathode of the tube Vl by a source Si and the other end coupled by resistance R. and inductance L to the control grid l. The control grid 8 in this embodiment receives its steady bias from source vSi and the potential drop due to grid rectification in resistance R. In this embodiment as in Fig. 1. the bypassing condenser 2 completes the radio frequency current path between the anode and control grid and the cathode.

The band pass filter including -X and N is tapped on coil Ll symmetrically each side of the center point connected by condenser 2 to ground. The filter of Fig. 2 is of the neutralized single crystal type. The neutralizing condenser N is made substantially identical to the crystal unit X electrically, except that the quartz plate'used in the neutralizing unit N is of a type that -will not oscillate. The crystal unit X is connected to the coil LI on the side of the grounded tap opposite to the side of LI to which the anode is connected. while the neutralizing unit N is connected to the anode side of the tank coil LI. The center connection between crystal unit X and the neutralizing unit N goes to the control grid of tube Vi through the phase shifting inductance coil L.

In operation of the circuit the tank circuit CILI is tuned to the filter carrier frequency when tube Vi is carrying unmodulated current. The filter frequency is determined in a large part by the dimensions of the crystal. The phase shifting elements are the inductance coil L and the parallel combination of the tube current resistance and the control grid to cathode tube capac-` ity C, as in Fig. l. As in Fig. 1, the inductive revactance of the phase shifting coil is made approximately equal to the capacity C at the mean frequency of operation, i. e., the carrier frequency. This provides optimum values of excitation voltage and phase shift of the feedback excitation supply to the grid 8.

When no modulation potentials are present the phase shift of the excitation reaching the grid 8 is constant because vthe tube current is constant. The tube and system oscillates at the steady carrier frequency. When modulation potentials or control potentials are applied to the grid 8 from source A through the transformer T the current in tube VI varies in accordance with the amplitude of the applied control or signal potentials. The tube resistance is varied thereby varying the phase of the excitation voltage on the control gr'id 8. This varies the generated frequency. For instance, an increase in the current lowers the tube resistance, produces a lag in the phase of the excitation voltage to decrease the frequency of the system. If the tube current is decreased the tube resistance increases. This causes a phase ,of the excitation voltage tobe advanced (lead) thereby increasing the frequency of operation of the system.

The crystal X and neutralizing condenser N acts as a band pass-filter which limits the amount of frequency modulation which can take place. The filter also stabilizes the carrier frequency of the oscillator. Its application would be most useful in frequency shift keying where small frequency deviations are used. The pur'- pose of the condenser N is to prevent the circuit from operating if the crystal X does not oscillate.

In the embodiment of Fig. 2, with the generated oscillations in the unmodulated condition aY carrier at 1770 kc. was obtained. Modulation of this carrier in various modifications produced total frequency variations from 140 cycles to 440 cycles. The circuit elements were about the same as for Fig. 1. The diilerent amounts of frequency-swing can probably be accounted for by the different adjustments of tank condenser' includes the winding I3 in series with source S2- and with the screen grid to cathode impedance of the tube. The polarity of the winding i with respect to the Winding i8 is as indicated, such as to modulate the screen grid l2 in the opposite direction with respect to the modulation on the control grid 8. This negative modulation, however, cannot be carried too far. Complete elimination of the undesired amplitude modulation by this opposed phase modulation of the screen grid might eliminate the frequency modulation of the generated oscillations in accordance with the control potentials.

When the arrangement is to send out frequency shift telegraphy signals modulating or keying means as shown' in Fig. 5 may be used. The generator and modulator tube here. is arranged and operated essentially as described in connection with Fig. 2.

However, the source A now supplies keying potentials to the potentiometer resistance R2 connected by resistance Rd to the grid of V3 and to ground and by cathode bias resistor Rl, shunted by a condenser of low impedance to potentials of the keying frequency, to the cathode of V3. The grid of V2 is connected to ground by the resistance R5 and potentiometer resistance R3. The latter resistance is tapped to switch S cooperating with two contacts l and 3 connected to the negative terminal of source B3, the positive terminal of which is connected to the cathode of tube V2.

The anodes and grids of V2 and V3 are crossconnected by resistances R6 and Rl. The anodes of tubes V2 and V3 are connected to resistances R9 and R8, the adjacent ends of which are connected to the direct current source. Potentiometer resistances Pi and P2 are connected between the anodes of tubes V2 and V3 and the taps on potentiometer resistances P3 and P3. These resistances are in shunt respectively to sources B1 and B2. A point on the potentiometer Pl is connected to the resistance R.- and thence to the control grid of tube Vi. A point on the potentiometer P2 is connected to the screen grid of the tube VI. A somewhatsimilar keyingtripping circuit is shown in Figs. 4 and 7 of my United States application #521,907, filed February 11, 1944, and in my United States Patent #2,326,314, dated August 10, 1943.

The keying-tripping circuit here, however, supplies negative bias to the control grid and positive bias to the screen grid. The tubes V2 and V3 are so operated that as keying potentials are applied the potentials at the anodes change diilerentially. If one tube, say V2, draws current, the drop in potential in R9 is impressed by the resistance R6 on the grid of V3 to reduce the current through V3. The rise in potential in R8 is applied to the grid of V2. This trips the current of the system through tube V2. When currentis Ainitiated in V3 a similar tripping operation takes place to trip the current through tube Vs.

The potentiometers R3' and R2 couple in the control or keying potentials. B3 is used when necessary to insure positive action of the tripping circuit.` The adjustment is such that the tube V2 is biased to cutoff in the absence of marking and spacing signals either by the drop in potential in resistances R5 and R3 or the same with negative potential from source B3 when switch Sis on the #i contact. The current then is switched through V3 and the drop in potentialv through R8 is applied by P2 to the screen grid l2 which now is made less positive. The increase in potential at the anode of V2 is applied from the anode end of R9 through resistance Pl to the control grid of the tube VI which is now made'less negative. Note that the control is diiTerential in that when the screen grid is made more positive the control grid is made more negative and vice versa. This would be the operation where tube V2 is cutoff and V3 draws current.

The keying potentials are applied to modulator tube Vl by the keying-tripping circuit of tubes V2 and V3. A steady negative bias is applied to the control grid of tube VI from source Bl through potentiometers P3 and Pi. Likewise, a steady positive bias is applied to the screen grid of tube VI from source B2 through potentiometers P3 and P2.. Both of these steady biases are adjustable by means of potentiometers P3 and Pil. As the keying source A operates the tripping circuit, the negative and positive biases on the control and screen grids of tube Vi are varied alternately in opposite directions. The amounts of these variations are adjustable by means of potentiometers Pl and P2. For example, if in the keying cycle tube V2 carries current and tube V3 is cut off the negative bias of the control grid of tube Vl will go more negative and the positive bias of the screen grid will go more positive. Again, if tube V3 carries current and tube V 2 is cut oi then the negative bias of the control grid of tube Vi will go less negative and the positive bias of the screen grid will go less positive, etc. This is in accordance with the part of the invention aimed to reduce amplitude modulation during frequency shift keying. It should be noted that if the sliding contact on Pl is raised up too far a positive bias will result. It would be possible to get, by this means, an alternating positive and negative bias at certain settings. However, it is generally desirable to have only a varying negative bias in this circuit. This would require that the sliding point on PI be kept down toward the lower or negative end of the potentiometer.

This biasing and keying arrangement may also be used in other modulator circuits where similar results are desired. f

The adjustment is such, however, that the keying potential'on .the control grid or screen grid keys the frequency of the generator from mark to space or vice versa while the differential control` on the screen grid or control grid compensates the undesired amplitude modulation. The keying system may operate to key only the screen grid or only the control grid by proper adjustment. Then either the connection from Pi to the control grid or from P2 to the screen grid is interrupted.

A more satisfactory method of eliminating the undesired amplitude modulation is to introduce some modulation on the screen grid cophasally with the modulation on the control grid and then to modulate the anode circuit in phase opposition with respect to that on the control grid. This method and means permits variation of the current past the control grid to produce the desired frequencymodulation but also maintains steady average current to the anode, thereby eliminating undesired amplitude modulation or variations.

An arrangement for carrying out this method of modulation is illustrated in Fig. 3. The embodiment in Fig. 3 is in general as described in Fig. 2. In Fig. 3, however, the screen grid l2 and the control grid 8 are cophasally modulated by potentials impressed on secondary windings I8 and i8. The anode is, in this modification, differentially or antiphasally modulated by potentials induced in the secondary winding 26, one end of which is connected to the lead to the midpoint on inductance Ll and thence to the anode of tube Vl. Note that in this embodiment the sources Si and S2, as in Fig. 2, supply the biasing potential for the grid and the charging potential for the screen grid respectively, while the source S3 supplies the direct current potential for the anode of the generator.

For optimum results to obtain the maximum frequency modulation with minimum amplitude modulation the potentials supplied by sources Si, S2 and S3 are to be of the correct values, and the transformer couplings between the primary winding and secondary I6 and between the primary winding and secondary 20 must be properly adjusted so that the cophasal modulation of the grid 8 and screen grid i2 produces the desired frequency variation, while the anti-phasal modulation of the anode 4 reduces the undesired amplitude modulation as much as possible without interfering with the operationnof the system. The adjustments of the potentials` from transformers I6, I8 and 20 may be accomplished by the use of potentiometers Pl', P2 and P3', respectively.

In all of the embodiments the condensers BP are large enough to shunt radio frequency potentials of the generated frequency around the secondary windings of the transformer and` the-bias or charging sources, but small enough to act as high impedances to potentials of the modulation frequency. l

I claim:

1. A signalling system comprising a Wave generator of the type wherein an electron discharge device has an anode, a cathode, and a control grid coupled regeneratively, for the generation of oscillations, by a tank circuit including reactance between the anode and control grid with a point on the reactance coupled to the cathode of the device, an inductance in series in the connection between the first mentioned reactance and the control grid of the device, said inductance being of a magnitude substantially equal to the reactance of the capacity between the control electrode and cathode of the device with respect to voltages of the frequency of the generated oscillations, an additional control electrode in said de vice, connections to said control grid for modulating the current through the tube in accordance with signals to correspondingly vary the phase of feedback and the frequency of the oscillations generated, and connections to said addithereto in phase opposition with respect to the control potentials japplied to said control grid to prevent undesired amplitude modulation of the generated oscillations.

2. A frequency modulator comprising an oscillation generator of the' type wherein an electron discharge tube having an anode, a cathode, and a control electrode has a tank circuit including reactance, regeneratively coupling saidclectrodes for the production of oscillations, there being a feedback coupling between said tank circuit and said control electrode, a grid bias resistance connecting said grid to the cathode of said tube, an inductance in said feedback coupling between the ta'nk circuit and the control grid, said inductance with the capacity between the control grid and cathode of the tube forming a phase shifting network, said grid bias resistance being connected to the end of said inductance remote from the control electrode to thereby exclude the bias resistance from the phase shifting network, and connections for varying the 'current through the tube in accordance with control potentials to correspondingly shift the phase of the feedback.

3. A wave length modulation system comprising an oscillation generator of the type wherein an electron discharge tube has a screening electrode and has an anode, a cathode, and a control electrode with connections regeneratively coupling said anode, cathode, and control electrode in an oscillation generator circuit including a tank circuit, there being in said connections a feedback coupling between said tank circuit and said control electrode, a reactance lin said feedback coupling between the tank circuit and the control electrode, a source of modulating potentials, connections for applying modulating potentials from said source to said screen grid and control electroie co-phasally, and connections from said source to said anode for applying thereto modulating potentials of a phase which is substantially opposed to the phase of the modulating potentials on said control electrode.

4. A signalling system comprising an oscillation generator of the type wherein an electron discharge device having a screen grid, an anode, a cathode, and a control grid, has its anode, cathode and control grid coupled regeneratively, for the production of oscillations, by a tank circuit including reactance between the anode and control grid with a point on the reactance coupled to the cathode of the device, an inductance in series in the connection between the first mentioned reactance and the control grid of the device, the reactance of said inductance being of a magnitude substantially equal to the reactance of the capacity between the control electrode and cathode of the'device with respect to oscillations of the frequency produced, connections to said control grid for modulating the current through the tube in accordance with signals, connections to said screen grid for applying control potentials thereto of a phase substantially the same as the phase of the potentials applied to said control grid, and connections to said anode for applying thereto control potentials of substantially opposed phase.

5. A frequency modulator comprising an oscillation generator of the type wherein an electron discharge tube having an anode, a cathode and a control electrode has a tank circuit including reactance regeneratively coupling said electrodes for the production 'of oscillations, there being a u feed back coupling between said tank circuit and tional control grid for applying control potentials 75 said control electrode, means for shifting the 6. A signalling system comprising an oscillation f generator of the type wherein an electron dis- -charge tube, having a screen grid, an anode, a cathode and a control electrode, has a tank circuit Y including reactance, regeneratively coupling said anode, cathode and control electrode for the production of oscillations, there being a feed back coupling between said tank circuit and said control electrode, a band pass filter and a phase shifting reactance in the feed back coupling between the tank circuit and the control electrode,

.said phase shifting reactance being between the band pass filter and the control electrode to cooperate with the reactance between the control electrode of the tube and its cathode to provide the phase shifting effect, a source of modulating potentials and a transformer having a primary winding coupled to said source of modulating potentials and having two secondary windings, one of which is coupled to said control electrode and the other of which is coupled to said screen grid for varying the potential on said control electrode to thereby vary the current through the tube in accordance with signals to correspondingly vary the tube reactance, the phase of feed back and the timing of the oscillations generated and to modulate the potential on said screen grid in a sense opposite to the modulation on said control electrode to compensate undesired amplitude modulation.

7. A frequency modulator comprising an oscillation generator of the type wherein an electron discharge tube having an anode, a cathode and a control electrode has a tank circuit including retrode, a band pass filter in said feed back coupling between the inductance therein and said tank circuit and connections to an electrode of the tube for varying the current through the tube in accordance with control potentials to vary the eifeciive resistance of the tube and the said tube re actance and correspondingly vary the phase of the feed back voltage.

8. A signalling system as recited in claim 6 wherein said band pass filter is a piezo-electric crystal.

9. A frequency modulator comprising an oscillation generator wherein an electron discharge tube having ascreen grid, an anode, a cathode and a control electrode has a tank circuit including reactance regeneratively coupling said electrodes for the production of oscillations, there being a feed back coupled between said tank circuit and said control electrode, and means for shifting the phase of feed back including the reactance between the control electrode and cathode of the tube and an inductance in said feed back coupling between the tank circuit and the control electrode, connections to the control electrode of the tube for varying the current through the tube in accordance with control p0- tentials to vary the effective resistance of the tube and the said tube reactance and correspondingly vary the phase of the feed back voltage and connections to said screen grid for applying control potentials thereto in phase opposition with respect to the control potentials on the control electrode to compensate undesired amplitude modulation produced in the frequency modulator.

` GEORGE L. USSELMAN.

REFERENCES CITED The following references are of record in the le of this patent:

STATES PATENTS Tuniek ...i June 26, 1945 

